Distributed acoustic sensing (DAS) technology has become a potential downhole acoustic logging tool for wellbore hydrocarbon fluid leakage detection and vertical seismic profiling, replacing more traditional single-point sensors such as hydrophone and geophone. This technology employs a fiber optic sensing cable disposed in the wellbore to listen to acoustic noise(s) that may arise from tubing, casing, cement, the casing annulus, the formation, wellbore fluids, wellbore equipment or other artificial sources. The optic fiber sensing cable functions as a continuous acoustic geophone or hydrophone for vertical seismic profiling, fracturing, and flow detections of which some flows could be leakage flows. However, use of DAS technology has been limited by the challenge of clearly interpreting measured acoustic data because of weak acoustic detection sensitivity of the sensing cable. One issue is that existing acoustic sensing cables do not effectively translate external acoustic field energy variations into measurable phase shifts because of ineffective strain coupling efficiency.
More specifically, DAS cables have to transfer wellbore structural deformation into fiber sensing cable elongation or compression, which leads to refractive indices variation in the fiber core. These elastic and optical strain effects can be measured and utilized to identify a parameter change in the vicinity of the localized strain. Known DAS cables have limited axial sensitivity due to strain-free package requirements and diminished transverse strain or radial sensitivity, and thus, are not well suited to detect weak acoustic signals that may signify a condition within the wellbore or surrounding formation. In other words, these cable package designs have only a limited range of detection sensitivity in longitudinal or axial direction. Despite great efforts made in system hardware, electronic circuits, modeling and signal processing, weak acoustic detection sensitivity is still one of the barriers preventing DAS technology from becoming a reliable and robust downhole acoustic logging tool.